For the past two decades, it is generally agreed that the most important development in nuclear medical imaging technology is the technique of positron emission tomography (PET), which is a nuclear medical imaging technique that produces a three-dimensional functional image or picture in the body. Operationally, a short-lived radioactive tracer isotope is injected into the living subject, usually into blood circulation through an intravenous glucose injection process, for allowing the radioactive tracer isotope to be chemically incorporated into malignant cancer cells. The radionuclide in the radiotracer decays and the resulting positrons subsequently annihilate on contact with electrons after traveling a short distance within the body. The encounter annihilates both electron and positron, producing a pair of annihilation (gamma) photons moving in approximately opposite directions, that is this electron-positron annihilation results in two 511 keV gamma photons (γ-ray) being emitted at almost 180 degrees to each other. Then, these pairs of annihilation (gamma) photons are collected by a PET scanner to be used in an image reconstruction process for obtaining molecular biology details of the living subject.
The count rate is an important factor regarding to the overall performance of a PET system, as a PET system with a high count rate, the imaging time required can be significantly reduced while still obtaining sufficient data. In addition, with the decreasing in imaging time, the acceptance of a patient to a PET scan procedure is increased as the pain and time required for the patient to remain still on a PET scanner is shortened significantly. Consequently, with the increasing amount of patients who are willing to take PET scanning, the amount of hospital willing to purchase the PET system is increased.
In a PET scanner, after an optical signal is converted into an electric signal by the use of scintillation crystal array in conjunction with photomultiplier tubes, the electric signal is fed to a circuit so as to produce a plurality of signals containing data relating to incident photon energy and location. Thereafter, the so-produced signals are sent to the integrators of the PET scanner so as to be used for determining values relating to incident photon energy according to the charge of capacitors in the integrators, and then the values are applied in a calculation so as to obtain the relative energy of incident photon and localize the annihilation event. However, after being charged, each integrator must be discharged before it can become available for processing next event, and thus, during the performing of a PET scan with high count rate, there can be a number of events that can be missed as they are happening during the discharging of integrator, which adversely impacts the count rate.
Conventionally, to reduce the effect of the dead time of the integrator on the overall system, there are generally multiple integrators to be used on each signal, as the two integrators shown in FIG. 1. In FIG. 1, a control unit 11 is enabled by an event triggering signal to issue integration control signals for controlling the charging/discharging of capacitors in corresponding integrators and a multiplexer control signal for controlling a multiplexer. According to the integration control signals from the control unit 11, a first integrator 12 or a second integrator 13 is selected to perform an integration to an input signal or to perform a discharging process while enabling the control unit 11 to issue a multiplexer control signal according to the selected integrator so as to control the multiplexer 14 to output signals from the selected integrator, which can be the first integrator 12 or the second integrator 13, as an output signal. Generally, the first integrator 12 and the second integrator 13 are selected alternately in a chronological order, as shown in FIG. 2. It is noted that the selection of integrators and the initiation of a integration period of a selected integrator is not enabled periodically, but is based upon the time when an event occurred. Therefore, the starting time of an integration period on a selected integrator must be controlled precisely and accurately to meet exactly to the time when an event occurred, and thus both the circuit architecture and the control method of the aforesaid integrators can be comparatively very complex.